The invention is a procedure for the manufacture of briquettes or pills for stoking made of stems from grain or straw or similar cellulosic waste material with the admixture of a binding agent during compression, principally by trace or piston pressing, at a pressure temporarily raising the temperature in the straw briquette to an area between 75.degree. C. and 200.degree. C.
Such procedures are known, e.g. from DK patent specification No. 148.119 describing a procedure for the manufacture of compressed straw briquettes for stoking, which is peculiar in that for its binding agent it uses hydrous sludge containing organic material, notably sludge from sewage purification or biological gas recovery. The advantage stated for this invention is especially the removal of sludge by means of straw briquettes and the exploitation of the calorific value of the organic content of the sludge, while the structure or deformation resistance of the briquette appear to be of less importance. Not surprisingly, such briquettes have proved unsuitable for transport because they lack deformation resistance properties. The briquettes frequently crumble at the smallest physical degree of stress, e.g. when stored in piles.
From DK patent application No. 90/84 it is known that a binding agent can be used for the stabilization of the deformation resistance of the briquette for the purpose of transport over relatively long distances. According to the latter application the binding agent is made up of fly ash, which, compared with other known binding agents is a cheap waste substance, which can even absorb a slightly increased moisture content in the straw. The drawbacks in using fly ash include the fact that the mineral content of fly ash involves a heavy mechanical wear on plungers and other parts in the manufacturing process and that the content of waste matter of the fly ash is not controllable with a view to substances polluting the environment. This, again, makes it inadvisable to use the ashes for fertilization.
From DK patent application No. 4471/81 we know of fuel briquettes or pills, which as binding agent uses the substance of tallbek in an amount corresponding to 1-10 weight percent. According to the application this binding agent should cause the effect that the briquette burns with a flame. Again, the drawback in this binding agent is that tallbek fails to ensure deformation resistance in the briquette sufficient enough to let the briquettes travel over long distances.
As tallbek is an oily product with a content of high fatty acids, resinous acids, esters and unsaponifiable substances, this matter does not correlate very well with the press during the manufacture of the briquettes or the pills, since tallbek, which only becomes thin at 80.degree. to 90.degree. C., glues together during cooling and deposits in the matrix and in the cooling stretch. Finally, it should be stated that tallbek is a material which has to be bought, which will often put a strain on a nation's balance of payments.
It is furthermore a known practice to use lignosulphonate under the trade name of WAFOLIN.RTM. as a binding agent. This substance produces poorly coherent briquettes, which easily crumble. Moreover, lignosulphonate has a sulphur content of about 5 percent so the use of lignosulphonate will result in an increased emission of sulphur.
A common feature of the known binding agents is that they all require a hydrous content between 12 and 16 percent in the straw, i.e. very dry straw. Under north-european climatic conditions drying to such a low hydrous content is seldom possible in the field before collecting the straw, and it has indeed turned out that most of the straw used for pressing briquettes has to be dehydrated before being used for briquettes. This dehydration process in itself is energy-consuming and comparatively cost-demanding so as to lower the total energy output of straw.
It is the object of the present invention to provide a procedure for the manufacture of briquettes or pills made of straw or similar cellulosic waste material with a hydrous content even greater than 16 percent using a binding agent to ensure a briquette of structural stability which will sustain transportation over long distances, and that this binding agent shall not have inherent secondary effects in the form of e.g. increased wear on plunger or matrix, neither should it contain environmental pollutants. Furthermore, together with the fuel, the binding agent must be capable of going into a cycle in agriculture, in which e.g. the ashes from the incineration can be used as a supplementary fertilizer.